A vast variety of technical applications rely on ultra thin multilayer materials to create key technical components.
For example, FIG. 1 shows an ultra thin membrane for hydrogen separation, which is built by depositing a thin layer of palladium (Pd) or a palladium alloy on a porous ceramic support. In this example, the rate of hydrogen transmission (throughput) through the membrane is proportional to the differential pressure ΔP and inversely proportional to the membrane thickness D. It is possible to significantly improve the throughput by increasing the differential pressure and decreasing the thickness of the membrane. A number of references, U.S. Pat. No. 6,810,899, U.S. Pat. No. 5,734,092, U.S. Pat. No. 5,652,020, U.S. Pat. No. 5,160,618, U.S. Pat. No. 4,857,080, incorporated herein by reference, disclose ultra thin membrane designs, with membrane thickness D ranging from 0.5 μm to 20 μm, which are able to sustain differential pressures up to about 1 atmosphere.
However, a possibility of mechanical failures at certain pressures limits the minimum membrane thickness. For example, U.S. Department of Energy research on a Membrane System for H2 Production under cooperative agreement No. DE-FC36-00GO10534 discloses a palladium membrane built on a porous substrate having an average pore size of less than 5 μm. The main disadvantage of this technique is in the fact that it is extremely difficult to completely exclude large outliers in pore distribution. In the above referenced case of a porous substrate, the authors report maximum pore size of 50 μm. This may lead to a mechanical breakdown and decreased separation factor for membranes with thickness less then 10 μm.
A 2004 National Energy Technology Laboratory research project entitled “Production of Pure Hydrogen from Hydrocarbons Using a Palladium Membrane Reactor”, funded by the Department of Energy, states the goal of attaining a Pd/Cu alloy membrane with thickness below 5 μm on porous ceramic supports. The publication also indicated that it is planned to achieve 1 μm thickness for Pd/Cu alloys on Group V-b metal foils.
An alternative approach, disclosed in the U.S. Pat. No. 6,810,899, suggests building perforated support plates for ultra thin membranes. The perforation is achieved by applying lithography methods to make small holes in the support structures. In this approach, the support structure has a very high aspect ratio (the ratio of the hole depth to the hole diameter) to achieve high durability and throughput. It is not a trivial exercise to build dense arrays of holes using existing lithography tools. Moreover, in order to push the membrane thickness below 0.1 μm, the hole diameter should be within the sub-100 nm range. It is very difficult to reliably fabricate such structures using existing lithography methods.
A wide variety of technology applications (defense, military, energy, automotive, etc.) create a demand to further miniaturize the membrane systems. There is also a need to further improve the productivity and reliability of the existing ultra thin membrane systems.